The basic form of controlled current source consists of an input transistor which is connected as a diode and of a usually identical output transistor with base and emitter terminals connected to the base and emitter terminals of the input transistor. During operation, the emitter terminals are both connected to a source of potential and the current through the collector of the output transistor is controlled by current applied to the common collector-base terminal connection of the input transistor. Input and output transistors are both of the NPN type or both of the PNP type. Therefore, the input and output currents both flow into the device or both flow out of the device in essentially a mirror image of each other. However, even when using identical transistors the D.C. input current differs in magnitude from the D.C. output current by the sum of the base currents of the transistors and the A.C. input current varies in both magnitude and phase from the A.C. output current because of the inherent parasitic and transit-time degradation.
Numerous improvements to the basic form of current controlled source have been proposed or embodied. Among the improvements are the so-called Wilson source, various cascade circuit arrangements and numerous circuit designs for supplying the base currents from sources other than the input current. Such improvements have not, in the majority of cases, resulted in improvement of frequency performance over that of the basic current mirror.
Current mirrors are commonly used in the design and fabrication of integrated circuits because of the inherently limited space requirements and the consequent necessity for elimination of large capacitance and resistance values from such circuits. Design preference is given to current mirrors fabricated using NPN transistors because of the vertical structure of the manufactured NPN transistor elements and the resultant large current gain and short transit time. However, where design requirements dictate use of PNP current mirrors and where cost factors prohibit use of fabrication techniques other than the conventional lateral structure of PNP transistor elements, compensation must be made for the low gain, the long transit time and the large value base currents. The circuit of this invention utilizes an unconventional method for deriving mirror currents and provides means for compensation and minimization of errors in offset, magnitude and phase between input and output currents.